Electronic memories are significantly important in more recent electronics circuits. Today, there are very few electronic circuits which do not incorporate one or more electronic memory circuits for the purpose of storing the microcode or the data to process.
The integration of a memory circuit in a more general microelectronic chip requires the qualification of the memory, which includes characterization, to precisely determine the internal parameters of the memory circuits which closely depend on the manufacturing process. Such parameters include the access time, the setup time and the hold time.
Methods are known for accessing the internal parameters of a memory circuit. A first method is known as “DUMMY PATH”. This method involves measuring the access time of a memory 10 to be qualified using the arrangement of two different transit paths for data between an input terminal 11(A) and an output terminal 13(B), as shown in FIG. 1. A multiplexer 12 allows the selection of one of the two paths for the purpose of measuring the transit time and thus, by computing the difference between the two measurements, the access time of the memory circuit 10 can be evaluated.
This first known method unfortunately has a significant drawback due to the low accuracy when operating at high frequencies. Indeed, it proves to be difficult to evaluate the time of transit of information through a memory circuit when this circuit is located within a semiconductor device that includes many different circuits, logical gates, multiplexers, etc., which all introduce their own internal delay in the processing time and, therefore, limit the accuracy of the measurement being made. Generally speaking, it is particularly tricky to accurately measure the time of transit of the information through the “sole” memory unit located within a whole complex micro-electronic chip.
A second known method is using “Built-In Self Test” (BIST) or “Built-In Self Characterization” (BISC) circuits which allow the measurement of some parameters of a memory circuit.
FIG. 2 shows the principle of a BISC-type circuit 20 which is located near a memory circuit 10 which has to be qualified. The BISC circuit generates a clock signal CK that is supplied to the memory 10 and receives back the data Q which is read from that memory. A complex processing is carried out to allow the qualification of some internal parameters of the memory to be achieved, particularly the access time and the setup time.
Known BISC circuits allow an improved accuracy with respect to the “DUMMY PATH” method, but necessitate the use of complex circuits that require significant space on the semiconductor substrate and this limits their use in complex semiconductor devices that are already filled with many internal circuits. At the same time, the need for qualifying a memory is particularly relevant in such complex semiconductor devices.
This is the reason why BISC circuits are generally used only for preproduction memories that are manufactured only for the purpose of the test and qualification process. These BISC circuits allow the internal parameters of the preproduction memories to be widely and extensively tested before starting mass production of customer memories which do not incorporate the BISC circuits. However, this solution is clearly not satisfactory.